A calcium carbonate nanofluid, its preparation method and application

Abstract

This invention provides a calcium carbonate nanofluid, its preparation method, and its application, belonging to the field of functional materials technology. The invention involves mixing hydrolyzed polymaleic anhydride, water, and an alkaline solution, followed by a neutralization reaction. This converts the carboxyl groups in the hydrolyzed polymaleic anhydride into carboxylate groups, facilitating subsequent chelation with calcium ions. A soluble calcium salt solution and a soluble carbonate solution are then added dropwise to the mixed solution to initiate a chelation precipitation reaction. During this reaction, calcium ions chelate with the hydrolyzed polymaleic anhydride and react with carbonate ions to form calcium carbonate, thereby forming a calcium carbonate nanofluid. This simultaneous chelation precipitation reaction improves the stability of the nanofluid, which also exhibits good interfacial tension reduction and emulsifying properties. When used as an oil displacement agent in oilfield development, it demonstrates excellent reservoir recovery.

Description

Technical Field

[0001] This invention belongs to the field of functional materials technology, specifically relating to a calcium carbonate nanofluid, its preparation method, and its application. Background Technology

[0002] Low-permeability reservoirs possess abundant reserves globally and exhibit significant production enhancement potential, thus gradually becoming a key focus of research and development in the oil and gas industry. Nanofluid displacement technology has been widely applied to improve reservoir recovery in low-permeability reservoirs. Currently, nanofluids encounter some difficulties in production enhancement, such as poor stability and high cost.

[0003] To address the issues of poor stability and high cost of nanofluids, many researchers have made numerous attempts. For example, Liang et al. prepared amphoteric nanosheets with good stability using octylamine-modified molybdenum disulfide, thus preparing AP-MDN nanofluids and achieving a reservoir recovery rate of 19.1%. Jia et al. proposed an innovative composite membrane nanofluid system, combining modified bacterial nanofibers with boron nitride nanosheets to increase the surface charge density of the nanosheets, thereby preparing stable nanofluids. Li et al. prepared 8-CD-SiO2 nanofluids using 8-cyclodextrin (8-CD)-grafted SiO2 nanoparticles, demonstrating excellent stability and salt resistance. Morteza Mansouri Zadeh et al. synthesized stable silica nanoparticles in the aqueous phase via a sol-gel process, increasing secondary oil recovery by 13.7%. Liu et al. selected the biocompatible reducing agent "ascorbic acid" to prepare nanosilver, and synthesized a bio-derived nanosilver complex using fungal fermentation supernatant, enhancing oil recovery. Sumadi Paryoto et al. prepared stable Fe3O4 nanofluids using composite surfactants and Fe3O4 nanoparticles. BilaA et al. attempted to stabilize nano-SiO2 by bonding or coating macromolecules (such as polymers) onto the surface of nanoparticles or modifying nanoparticles. Sagala et al. prepared stable nanofluids using surfactant-coated nanomaterials. Bhuvanesh et al. prepared AGO / HPAM nanofluids using different concentrations of AGO nanocomposites, which improved reservoir recovery by 15.7% compared to pure polymer displacement. Li et al. synthesized carbon nanodots (CDs) via hydrothermal reaction and prepared activated carbon nanodots (sm-CDs) by surface modification, enhancing oil recovery. Tian et al. prepared amphoteric titanium dioxide nanofluids by chemically reacting nano-titanium dioxide (NTD) with a titanium ester coupling agent, and dispersed them using Span85 dispersant; the nanofluid displacement technology improved reservoir recovery by 26.4%. Zhao et al. prepared a stable functional silicon nanofluid that could withstand high-salt and high-temperature environments by using hydrophilic silica sol with 0.1 wt% of the modifier lauramide propyl hydroxysulfonyl butanediol.

[0004] The methods described above can be mainly divided into two categories. One category uses surfactants or polymers as stabilizers to prepare stable nanofluids. The other category modifies the surface of nanoparticles to give them the same surface charge, utilizing electrostatic repulsion to stabilize the nanofluids. However, the stability of nanofluids prepared by these methods is still relatively poor. Therefore, there is an urgent need for a method for preparing nanofluids with good stability. Summary of the Invention

[0005] The purpose of this invention is to provide a calcium carbonate nanofluid, its preparation method, and its applications. The calcium carbonate nanofluid prepared by this invention exhibits excellent stability.

[0006] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0007] This invention provides a method for preparing calcium carbonate nanofluids, comprising the following steps:

[0008] (1) Hydrolyzed polymaleic anhydride, water and alkaline solution are mixed and neutralized to obtain a mixed solution;

[0009] (2) Add the soluble calcium salt solution and the soluble carbonate solution dropwise to the mixed solution obtained in step (1) to carry out a chelation precipitation reaction and obtain calcium carbonate nanofluid.

[0010] Preferably, in step (1), the volume ratio of hydrolyzed polymaleic anhydride to water is (0.01-0.2):1.

[0011] Preferably, the pH value of the mixed system obtained by mixing water and alkali solution in step (1) is 9 to 13.

[0012] Preferably, the soluble calcium salt solution in step (2) includes one or more of calcium chloride aqueous solution, calcium nitrate aqueous solution and calcium acetate aqueous solution.

[0013] Preferably, the soluble carbonate solution in step (2) includes one or both of sodium carbonate aqueous solution and potassium carbonate aqueous solution.

[0014] Preferably, the concentration of the soluble calcium salt solution in step (2) is 0.5 to 2 mol / L.

[0015] Preferably, the volume ratio of the soluble calcium salt solution in step (2) to the hydrolyzed polymaleic anhydride in step (1) is (1-20):1.

[0016] Preferably, in step (2), the molar ratio of calcium ions in the soluble calcium salt solution to carbonate ions in the soluble carbonate solution is (0.9-1.1):1.

[0017] The present invention also provides calcium carbonate nanofluid prepared by the preparation method described in the above technical solution.

[0018] The present invention also provides the application of the calcium carbonate nanofluid described in the above technical solution in oilfield exploitation.

[0019] This invention provides a method for preparing calcium carbonate nanofluid, comprising the following steps: (1) mixing hydrolyzed polymaleic anhydride, water, and alkaline solution for neutralization reaction to obtain a mixed solution; (2) adding a soluble calcium salt solution and a soluble carbonate solution dropwise to the mixed solution obtained in step (1) for chelation precipitation reaction to obtain calcium carbonate nanofluid. In this invention, the hydrolyzed polymaleic anhydride, water, and alkaline solution are mixed and then neutralized to convert the carboxyl groups in the hydrolyzed polymaleic anhydride into carboxylate groups, facilitating subsequent chelation with calcium ions. The soluble calcium salt solution and the soluble carbonate solution are then added dropwise to the mixed solution for chelation precipitation reaction. During this reaction, calcium ions chelate with the hydrolyzed polymaleic anhydride on one hand, and react with carbonate groups on the other hand to form calcium carbonate, thereby forming calcium carbonate nanofluid. Through simultaneous chelation precipitation reaction, the stability of the nanofluid is improved, and the nanofluid exhibits good interfacial tension reduction and emulsification properties. When used as an oil displacement agent in oilfield exploitation, it has excellent reservoir recovery rate. The results of the examples show that the calcium carbonate nanofluid prepared by the present invention still has 46% unprecipitated after standing for 10 days, while the pure calcium carbonate nanofluid is completely precipitated on the first day, and the nanofluid obtained by directly mixing calcium carbonate and hydrolyzed polymaleic anhydride is also completely precipitated after stirring is stopped. Attached Figure Description

[0020] Figure 1 This is a schematic flowchart of the preparation method of calcium carbonate nanofluid in Example 1 of the present invention;

[0021] Figure 2 This is a diagram illustrating the stability mechanism of the calcium carbonate nanofluid in Example 1 of the present invention;

[0022] Figure 3 The infrared spectra of the calcium carbonate nanofluid prepared in Example 1 and the calcium carbonate nanofluid prepared in Comparative Example 1 are shown below.

[0023] Figure 4 Thermogravimetric curves of the calcium carbonate nanofluid prepared in Example 1 and the calcium carbonate nanofluid prepared in Comparative Example 1 are shown.

[0024] Figure 5 The oil-water interfacial tension diagrams are shown for the calcium carbonate nanofluid prepared in Example 1 of this invention, the calcium carbonate nanofluid prepared in Comparative Example 1, and deionized water.

[0025] Figure 6 The diagram shows the emulsification index of the calcium carbonate nanofluid prepared in Example 1 of this invention at different concentrations.

[0026] Figure 7 This is a macroscopic image of the calcium carbonate nanofluid prepared in Example 1 of the present invention after standing at room temperature for 1 to 10 days;

[0027] Figure 8Macroscopic images of the calcium carbonate nanofluid prepared in Comparative Example 1 after standing at room temperature for 1–10 days.

[0028] Figure 9 Stability diagrams of the calcium carbonate nanofluid prepared in Example 1 and the calcium carbonate nanofluid prepared in Comparative Example 2 of this invention.

[0029] Figure 10 This is an experimental diagram of the displacement of calcium carbonate nanofluids prepared in Example 1 of the present invention;

[0030] Figure 11 This is a diagram illustrating the mechanism by which calcium carbonate nanofluid prepared in Example 1 of this invention enhances oil recovery. Detailed Implementation

[0031] This invention provides a method for preparing calcium carbonate nanofluids, comprising the following steps:

[0032] (1) Hydrolyzed polymaleic anhydride, water and alkaline solution are mixed and neutralized to obtain a mixed solution;

[0033] (2) Add the soluble calcium salt solution and the soluble carbonate solution dropwise to the mixed solution obtained in step (1) to carry out a chelation precipitation reaction and obtain calcium carbonate nanofluid.

[0034] Unless otherwise specified, the present invention does not impose any special restrictions on the source of the raw materials, and commercially available products well known to those skilled in the art can be used.

[0035] This invention involves mixing hydrolyzed polymaleic anhydride, water, and alkaline solution, followed by a neutralization reaction to obtain a mixed solution.

[0036] In this invention, the average molecular weight of the hydrolyzed polymaleic anhydride is preferably 400-600, more preferably 500. By limiting the average molecular weight of the hydrolyzed polymaleic anhydride to the above range, this invention enables it to have a suitable carbon chain length, better steric hindrance effect, and further improve the stability of calcium carbonate nanofluids.

[0037] In this invention, the volume ratio of the hydrolyzed polymaleic anhydride to water is preferably (0.01–0.2):1, more preferably (0.02–0.15):1. By limiting the volume ratio of the hydrolyzed polymaleic anhydride to water within the above range, this invention ensures that the raw materials are fully dissolved.

[0038] In this invention, the alkaline solution preferably comprises an aqueous solution of sodium hydroxide or an aqueous solution of potassium hydroxide. In this invention, the concentration of the alkaline solution is preferably 1–3 mol / L, more preferably 2 mol / L.

[0039] In this invention, the preferred method for mixing the hydrolyzed polymaleic anhydride, water, and alkaline solution is to mix water and alkaline solution to obtain a mixed system, and then add the hydrolyzed polymaleic anhydride.

[0040] In this invention, the pH value of the mixture obtained by mixing water and alkali solution is preferably 9-13, more preferably 10-13. This invention does not have a specific limitation on the amount of alkali solution used, as long as the pH value of the mixture is within the above range. Limiting the pH value of the mixture to the above range in this invention is more conducive to the hydrolysis of carboxyl groups in polymaleic anhydride into carboxylate groups, and more conducive to subsequent chelation with calcium ions.

[0041] In this invention, the preferred temperature for the neutralization reaction is 20–30°C. In this invention, the neutralization reaction begins upon the addition of hydrolyzed polymaleic anhydride, and the process of adding and mixing the hydrolyzed polymaleic anhydride constitutes the neutralization reaction process. In this invention, during the neutralization reaction, the carboxyl groups in the hydrolyzed polymaleic anhydride react under alkaline conditions to form carboxylate ions.

[0042] After obtaining the mixed solution, the present invention adds a soluble calcium salt solution and a soluble carbonate solution dropwise to the mixed solution to carry out a chelation precipitation reaction, thereby obtaining calcium carbonate nanofluid.

[0043] In this invention, the soluble calcium salt solution preferably includes one or more of calcium chloride aqueous solution, calcium nitrate aqueous solution and calcium acetate aqueous solution, and more preferably calcium chloride aqueous solution.

[0044] In this invention, the concentration of the soluble calcium salt solution is preferably 0.5–2 mol / L, more preferably 1 mol / L. By limiting the concentration of the soluble calcium salt solution within the above range, this invention enables the calcium salt to dissolve sufficiently and subsequently exhibits a suitable chelation precipitation rate, further improving the stability of the calcium carbonate nanofluid.

[0045] In this invention, the volume ratio of the soluble calcium salt solution to the hydrolyzed polymaleic anhydride is preferably (1-20):1, more preferably 10:1. By limiting the volume ratio of the soluble calcium salt solution to the hydrolyzed polymaleic anhydride within the above range, this invention enables a greater amount of hydrolyzed polymaleic anhydride to be chelated on the surface of calcium carbonate, thereby further improving the stability of the calcium carbonate nanofluid.

[0046] In this invention, the soluble carbonate solution preferably includes one or both of sodium carbonate aqueous solution and potassium carbonate aqueous solution, more preferably sodium carbonate aqueous solution.

[0047] In this invention, the concentration of the soluble carbonate solution is preferably 0.5–2 mol / L, more preferably 1 mol / L. By limiting the concentration of the soluble carbonate solution within the above range, this invention enables the carbonate to dissolve fully and subsequently exhibits a suitable chelation precipitation rate, further improving the stability of the calcium carbonate nanofluid.

[0048] In this invention, the volume ratio of the soluble carbonate solution to the hydrolyzed polymaleic anhydride is preferably (1-20):1, more preferably 10:1. By limiting the volume ratio of the soluble carbonate solution to the hydrolyzed polymaleic anhydride within the above range, this invention enables a greater amount of hydrolyzed polymaleic anhydride to chelate on the surface of calcium carbonate, thereby further improving the stability of the calcium carbonate nanofluid.

[0049] In this invention, the volume ratio of the soluble calcium salt solution to the soluble carbonate solution is preferably (0.9–1.1):1, more preferably 1:1; the molar ratio of calcium ions in the soluble calcium salt solution to carbonate ions in the soluble carbonate solution is preferably (0.9–1.1):1, more preferably 1:1. By limiting the molar ratio of calcium ions in the soluble calcium salt solution to carbonate ions in the soluble carbonate solution to the above range, this invention ensures that the two react fully to form calcium carbonate.

[0050] In this invention, the dropping rates of the soluble calcium salt solution and the soluble carbonate solution are preferably the same. Specifically, the dropping rate of the soluble calcium salt solution and the soluble carbonate solution is preferably 0.5–5 mL / min. This invention, by simultaneously adding the soluble calcium salt solution and the soluble carbonate solution and limiting their dropping rates, allows the chelation and precipitation reactions to proceed fully, further improving the stability of the calcium carbonate nanofluid.

[0051] In this invention, the soluble calcium salt solution and the soluble carbonate solution are preferably added dropwise in constant-pressure funnels. Using constant-pressure funnels facilitates the dropwise addition process.

[0052] In this invention, the temperature of the chelation precipitation reaction is preferably 40–100°C, more preferably 60–80°C; the time of the chelation precipitation reaction is preferably 1–24 h, more preferably 1–5 h; the chelation precipitation reaction is preferably carried out under stirring conditions; the stirring rate is preferably 700–900 r / min, more preferably 800 r / min. In this invention, the chelation precipitation reaction begins upon the addition of the soluble calcium salt solution and the soluble carbonate solution. In this invention, during the chelation precipitation reaction, calcium ions chelate with hydrolyzed polymaleic anhydride on one hand, and react with carbonate ions on the other hand to form calcium carbonate, thereby forming calcium carbonate nanofluid. By limiting the temperature and time of the chelation precipitation reaction within the above-mentioned ranges, this invention enables a more complete reaction.

[0053] After the chelation precipitation reaction is completed, the product of the chelation precipitation reaction is preferably cooled to obtain calcium carbonate nanofluid.

[0054] The present invention does not impose any special limitations on the cooling operation; any cooling technique known to those skilled in the art can be used to cool the material to room temperature.

[0055] This invention involves mixing hydrolyzed polymaleic anhydride, water, and alkaline solution, followed by a neutralization reaction. This transforms the carboxyl groups in the hydrolyzed polymaleic anhydride into carboxylate groups, facilitating subsequent chelation with calcium ions. A soluble calcium salt solution and a soluble carbonate solution are then added dropwise to the mixed solution to initiate a chelation precipitation reaction. During this reaction, calcium ions chelate with the hydrolyzed polymaleic anhydride and react with carbonate ions to form calcium carbonate, thereby forming calcium carbonate nanofluid. This simultaneous chelation precipitation reaction enhances the stability of the nanofluid. Furthermore, the nanofluid exhibits good interfacial tension reduction and emulsifying properties, resulting in excellent reservoir recovery when used as an oil displacement agent in oilfield development.

[0056] The present invention also provides calcium carbonate nanofluid prepared by the preparation method described in the above technical solution.

[0057] The calcium carbonate nanofluid prepared by this invention has excellent stability, interfacial tension reduction performance and emulsifying properties, and exhibits excellent reservoir recovery when used as an oil displacement agent in oilfield development.

[0058] The present invention also provides the application of the calcium carbonate nanofluid described in the above technical solution in oilfield exploitation.

[0059] The present invention does not impose any special limitations on the operation of the application, and any technical solution known to those skilled in the art can be used.

[0060] The technical solutions of this invention will be clearly and completely described below with reference to the embodiments thereof. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0061] Example 1

[0062] (1) At room temperature, add 200 mL of deionized water to a three-necked flask, add 20 mL of 2 mol / L sodium hydroxide aqueous solution to make the pH of the mixed system 13, and then add 5 mL of hydrolyzed polymaleic anhydride (average molecular weight 500) to obtain a mixed solution (the volume ratio of hydrolyzed polymaleic anhydride to deionized water is 0.025:1).

[0063] (2) Place 50 mL of 1 mol / L calcium chloride aqueous solution and 50 mL of 1 mol / L sodium carbonate aqueous solution in constant pressure funnels, and then place them in the two necks of a three-necked flask. Add calcium chloride aqueous solution and sodium carbonate aqueous solution to the three-necked flask at the same rate (dropping rate 1 mL / min, volume ratio of calcium chloride aqueous solution to sodium carbonate aqueous solution 1:1, molar ratio of calcium ions in calcium chloride aqueous solution to carbonate ions in sodium carbonate aqueous solution 1:1, volume ratio of calcium chloride aqueous solution to hydrolyzed polymaleic anhydride 10:1). React at 60℃ for 1 h with stirring at a stirring rate of 800 r / min. Cool to obtain calcium carbonate nanofluid, denoted as CaCO3-HPMA.

[0064] Example 2

[0065] (1) At room temperature, add 80 mL of deionized water to a three-necked flask, add 6 mL of 2 mol / L sodium hydroxide aqueous solution to make the pH of the mixed system 12.5, and then add 6 mL of hydrolyzed polymaleic anhydride (average molecular weight 500) to obtain a mixed solution (the volume ratio of hydrolyzed polymaleic anhydride to deionized water is 0.075:1).

[0066] (2) Place 50 mL of 1 mol / L calcium chloride aqueous solution and 50 mL of 1 mol / L sodium carbonate aqueous solution in constant pressure funnels, and then place them in the two necks of a three-necked flask. Add calcium chloride aqueous solution and sodium carbonate aqueous solution to the three-necked flask at the same rate (dropping rate 0.5 mL / min, volume ratio of calcium chloride aqueous solution to sodium carbonate aqueous solution 1:1, molar ratio of calcium ions in calcium chloride aqueous solution to carbonate ions in sodium carbonate aqueous solution 1:1, volume ratio of calcium chloride aqueous solution to hydrolyzed polymaleic anhydride 8.3:1). React at 60℃ for 1 h with stirring at a stirring rate of 800 r / min. Cool to obtain calcium carbonate nanofluid.

[0067] Example 3

[0068] (1) At room temperature, add 100 mL of deionized water to a three-necked flask, add 9 mL of 2 mol / L sodium hydroxide aqueous solution to make the pH of the mixed system 13, and then add 6 mL of hydrolyzed polymaleic anhydride (average molecular weight 500) to obtain a mixed solution (the volume ratio of hydrolyzed polymaleic anhydride to deionized water is 0.06:1).

[0069] (2) Place 50 mL of 1 mol / L calcium chloride aqueous solution and 50 mL of 1 mol / L sodium carbonate aqueous solution in constant pressure funnels, and then place them in the two necks of a three-necked flask. Add calcium chloride aqueous solution and sodium carbonate aqueous solution to the three-necked flask at the same rate (dropping rate 2.0 mL / min, volume ratio of calcium chloride aqueous solution to sodium carbonate aqueous solution 1:1, molar ratio of calcium ions in calcium chloride aqueous solution to carbonate ions in sodium carbonate aqueous solution 1:1, volume ratio of calcium chloride aqueous solution to hydrolyzed polymaleic anhydride 8.3:1). React at 70℃ for 1 h with stirring at a stirring rate of 800 r / min. Cool to obtain calcium carbonate nanofluid.

[0070] Comparative Example 1

[0071] The hydrolysis of polymaleic anhydride in Example 1 was omitted, and calcium carbonate nanofluid was obtained, denoted as CaCO3.

[0072] Comparative Example 2

[0073] The calcium carbonate nanofluid prepared in Comparative Example 1 was mixed with hydrolyzed polymaleic anhydride (the mass ratio of calcium carbonate to hydrolyzed polymaleic anhydride was 1:1) to obtain a calcium carbonate nanofluid with a concentration of 15 g / L.

[0074] Example 1: A schematic flowchart of the preparation method of calcium carbonate nanofluid is preferably shown below. Figure 1 As shown.

[0075] The stability mechanism diagram of the calcium carbonate nanofluid prepared in Example 1 is shown below. Figure 2 As shown, the excellent stability of CaCO3-HPMA nanofluids is due, on the one hand, to the significant steric hindrance of hydrolyzed polymaleic anhydride, which partially offsets van der Waals attraction; and on the other hand, to the negative charge of sodium carboxylate in aqueous solution. The electrostatic repulsion of CaCO3-HPMA is sufficient to counteract aggregation caused by van der Waals forces.

[0076] The infrared spectra of the CaCO3-HPMA nanofluid prepared in Example 1 and the CaCO3 nanofluid prepared in Comparative Example 1 are as follows: Figure 3 As shown. From Figure 3 As can be seen from this, 713.2cm -1 and 858.1cm -1 The peak was attributed to the bending vibration of the CO bond, 1090.1 cm⁻¹. -1 and 1477.2cm -1 The peak corresponds to the symmetric and antisymmetric stretching vibrations of the CO bond, at 3432.6 cm⁻¹. -1 The peak at 2929.3 cm⁻¹ represents the stretching vibration of hydroxyl groups originating from bound water. -1 and 2850.2cm -1The peak represents the stretching vibration of the C-C bond, while 1798.9 cm⁻¹ -1 and 1710.6cm -1 The peaks were identified as antisymmetric and symmetric stretching vibrations of the C=O bond. These results confirm the successful synthesis of CaCO3-HPMA.

[0077] The thermogravimetric curves of the CaCO3-HPMA nanofluid prepared in Example 1 and the CaCO3 nanofluid prepared in Comparative Example 1 are shown below. Figure 4 As shown. From Figure 4 As can be seen, the mass loss process can be divided into three stages. In the first stage, from 27 to 100℃, the mass loss rate of CaCO3-HPMA is 1.0%, and the mass loss rate of CaCO3 is 0.01%, mainly attributed to the evaporation of water molecules adsorbed on the surfaces of CaCO3 and CaCO3-HPMA. Compared with CaCO3, CaCO3-HPMA has more hydrophilic groups and can adsorb more water, thus leading to greater mass loss. In the second stage, from 100℃ to 530℃, the mass loss rate of CaCO3-HPMA is 14.2%, and that of CaCO3 is 0.0%. The significant increase in the mass loss of CaCO3-HPMA is attributed to the decomposition of HPMA grafted onto the surface of CaCO3-HPMA. When the temperature exceeds 530℃ and enters the third stage, the mass loss rate of CaCO3-HPMA is 10.3%, and that of CaCO3 is 15.2%. The significant increase in the mass loss of both CaCO3 and CaCO3-HPMA is attributed to the thermal decomposition of CaCO3.

[0078] The interfacial tension between the CaCO3-HPMA nanofluid prepared in Example 1 and the CaCO3 nanofluid prepared in Comparative Example 1 and deionized water was measured using the pendant drop method. CaCO3-HPMA nanofluid and CaCO3 nanofluid were dispersed in deionized water to prepare 1000 mg / L nanofluids, with kerosene as the oil phase. The interfacial tension between the nanofluid and kerosene was measured at 60°C. Measurements were taken every 10 seconds, and the steady-state interfacial tension values ​​were recorded over 600 seconds. The results are shown below. Figure 5 As shown. From Figure 5 It can be seen that the interfacial tension between oil and water decreases continuously with increasing time. At 600 s, the change in interfacial tension between oil and water is relatively small, indicating that the prepared CaCO3-HPMA nanofluid has a good performance in reducing interfacial tension.

[0079] The CaCO3-HPMA nanofluid prepared in Example 1 was dispersed in deionized water to prepare nanofluids of different concentrations (250 mg / L, 500 mg / L, and 1000 mg / L). 10 mL of the nanofluid and 10 mL of kerosene were added to a glass bottle, and emulsified using an emulsifier at 10000 rpm for 2 min. The mixture was then observed at 60°C. The emulsification index was the ratio of the emulsified volume to the total volume after standing for 24–240 h. A higher emulsification index indicates a better emulsification effect, as shown in the results. Figure 6 As shown. From Figure 6 As can be seen, the emulsifying properties of CaCO3-HPMA nanofluids increase with increasing concentration but decrease over time. For CaCO3-HPMA nanofluid (250 mg / L), the emulsification index is 10% after 240 h at 60℃. For CaCO3-HPMA nanofluid (500 mg / L), the emulsification index is 15% after 240 h at 60℃. For CaCO3-HPMA nanofluid (1000 mg / L), the emulsification index is 20% after 240 h at 60℃. This indicates that CaCO3-HPMA nanofluids possess excellent emulsifying properties.

[0080] The stability of the CaCO3-HPMA nanofluid (15 g / L) prepared in Example 1 and the CaCO3 nanofluid (15 g / L) prepared in Comparative Example 1 was evaluated by allowing them to stand at room temperature. The results are as follows: Figure 7 and Figure 8 As shown. From Figures 7-8 As can be seen, after standing at room temperature for 10 days, 46% of the CaCO3-HPMA nanofluid (15 g / L) remained unprecipitated, indicating its relatively good stability. In contrast, the CaCO3 nanofluid (15 g / L) completely precipitated on the first day.

[0081] The stability of the CaCO3-HPMA nanofluid (15 g / L) prepared in Example 1 and the CaCO3 nanofluid (15 g / L) prepared in Comparative Example 2 was evaluated by allowing them to stand at room temperature. The results are as follows: Figure 9 As shown, Figure 9 (a) is a macroscopic view of the CaCO3-HPMA nanofluid prepared in Example 1; (b) is a schematic diagram of the structure of the CaCO3-HPMA nanofluid prepared in Example 1; (c) is a macroscopic view of the CaCO3 nanofluid prepared in Comparative Example 2; and (d) is a schematic diagram of the structure of the CaCO3 nanofluid prepared in Comparative Example 2. Figure 9As can be seen, the nanofluid (concentration 15 g / L) prepared by mixing CaCO3 nanoparticles and HPMA in Comparative Example 2 has poor stability. After stirring was stopped, almost all the CaCO3 nanoparticles precipitated at the bottom of the graduated cylinder. This is mainly because there are very few calcium ions on the surface of the calcium carbonate nanoparticles, making it almost impossible for them to chelate with hydrolyzed polymaleic anhydride HPMA. The calcium carbonate nanoparticles settled under the influence of gravity. In contrast, the CaCO3 nanoparticles prepared in Example 1 of this invention... 2+ It will chelate with HPMA on one hand, and with CO3 on the other. 2- The reaction produces CaCO3. The two reactions were combined to prepare CaCO3-HPMA nanofluids. First, CaCO3-HPMA exhibits negative charge in aqueous solution, while CaCO3-HPMA carries the same charge. Second, the surface of CaCO3-HPMA grafted with HPMA has a certain steric hindrance, which can increase the stability of the CaCO3-HPMA nanofluids. The electrostatic repulsion force of CaCO3-HPMA stabilizes the nanofluids.

[0082] Application examples

[0083] To explore the ability of the CaCO3-HPMA nanofluid prepared in Example 1 to improve the permeability of low-permeability reservoirs, an oil displacement experiment was conducted. The permeability of the simulated reservoir was 15.2 × 10⁻⁶. -3 μm 2 The core parameters and the results of enhancing oil recovery using the CaCO3-HPMA nanofluid prepared in Example 1 are shown in Table 1 and... Figure 10 As shown in the diagram, the mechanism of CaCO3-HPMA nanofluid enhancing oil recovery is as follows: Figure 11 As shown.

[0084] Table 1. Core parameters and results of enhanced oil recovery from CaCO3-HPMA nanofluid prepared in Example 1.

[0085]

[0086] From Table 1 and Figure 10It is observed that the injection pressure in the simulated formation rapidly increases after the first waterflood. After the injection of CaCO3-HPMA nanofluid, the injection pressure decreases, and after the second waterflood, the injection pressure increases slightly before stabilizing at 3.8 MPa. Injecting 0.5 PV of CaCO3-HPMA nanofluid, combined with the subsequent second waterflood, increases the recovery rate by 14.0%. This indicates that CaCO3-HPMA nanofluid flooding can improve the recovery rate of low-permeability reservoirs. The water cut continuously increases after the first waterflood, but decreases significantly after CaCO3-HPMA nanofluid flooding, further demonstrating that CaCO3-HPMA nanofluid can improve the recovery rate of medium-to-low permeability reservoirs. This is partly due to the wedge-shaped region formed at the rock / crude oil / injection fluid three-phase interface during the flooding process, utilizing the wedge-shaped compression effect to improve crude oil recovery. Secondly, the amphiphilic nature of CaCO3-HPMA reduces the interfacial tension between oil and water, emulsifies the crude oil, and transforms the oil-wetted reservoir into a water-wetted reservoir, thus facilitating waterflooding.

[0087] CaCO3-HPMA has an amphiphilic structure, a large specific surface area, and high surface energy, which allows it to be better adsorbed at the oil-water interface to form a thin film. Figure 11 This study demonstrates the mechanism by which CaCO3-HPMA nanofluids enhance oil recovery efficiency, which can be attributed to four key factors. First, the reservoir rock surface is primarily oleophilic or mixed-wetness. When the CaCO3-HPMA nanofluid contacts the rock, it transforms the surface into a hydrophilic or neutral wettability. This change reduces resistance to oil droplet movement, facilitating waterflooding and improving oil recovery efficiency. Second, a wedge-shaped region is formed at the interface between the rock, crude oil, and injected fluid. This region generates a forward force through Brownian motion and electrostatic interactions, effectively stripping crude oil from the rock and improving oil recovery efficiency. Third, the CaCO3-HPMA nanofluid reduces the interfacial tension between oil and water, aiding in oil film detachment. Fourth, the CaCO3-HPMA nanofluid emulsifies the crude oil, breaking it down into smaller droplets that are more easily carried away.

[0088] In summary, the calcium carbonate nanofluid prepared by this invention has excellent stability, and the nanofluid has good performance in reducing interfacial tension and emulsifying properties. When used as an oil displacement agent in oilfield development, it has excellent reservoir recovery rate.

[0089] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing calcium carbonate nanofluid, comprising the following steps: (1) Hydrolyzed polymaleic anhydride, water and alkaline solution are mixed and neutralized to obtain a mixed solution; (2) Add the soluble calcium salt solution and the soluble carbonate solution dropwise to the mixed solution obtained in step (1) to carry out a chelation precipitation reaction and obtain calcium carbonate nanofluid; In step (1), the volume ratio of hydrolyzed polymaleic anhydride to water is (0.01~0.2):1; the pH value of the mixed system obtained by mixing water and alkali solution in step (1) is 9~13; the concentration of soluble calcium salt solution in step (2) is 0.5~2mol / L; the volume ratio of soluble calcium salt solution in step (2) to hydrolyzed polymaleic anhydride in step (1) is (1~20):1; the molar ratio of calcium ions in soluble calcium salt solution to carbonate ions in soluble carbonate solution in step (2) is (0.9~1.1):1; The average molecular weight of the hydrolyzed polymaleic anhydride is 400-600. The chelation precipitation reaction is carried out at a temperature of 40~100℃; the chelation precipitation reaction is carried out for a time of 1~24h; the chelation precipitation reaction is carried out under stirring conditions; the stirring rate is 700~900r / min.

2. The preparation method according to claim 1, characterized in that, The soluble calcium salt solution in step (2) includes one or more of calcium chloride aqueous solution, calcium nitrate aqueous solution and calcium acetate aqueous solution.

3. The preparation method according to claim 1, characterized in that, The soluble carbonate solution in step (2) includes one or both of sodium carbonate aqueous solution and potassium carbonate aqueous solution.

4. The calcium carbonate nanofluid prepared by the preparation method according to any one of claims 1 to 3.

5. The application of the calcium carbonate nanofluid according to claim 4 in oilfield exploitation.

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